The invention relates according to the generic part of claim 1 to a method for producing spectacle lenses from raw lenses prefabricated in a non-cutting manner. The raw lenses have a central axis standing vertically on them and comprise a convex front side that is formed in a rotationally symmetrical manner about the central axis and is not to be reworked and comprises an opposing back side that is to be reworked with a CNC machine. The central axis is a purely geometric axis and is not actually physically present. Front- and back side have at least one optical focal point in common. Furthermore, a prescription describes a visual error of a prescription holder. In order to rework the back side a prescription surface is calculated on the basis of the prescription and of the geometry of the front side. This surface forms a prescription lens in common with the front side, the focal points of which lens correct the visual error of the prescription holder. A spectacle lens with an optical center is produced from the prescription lens by adaptation to an eyeglass frame through which an eyeglasses wearer looks when looking straight ahead. This view looking straight ahead is also designated as the zero view direction.
Spectacle lenses serve as a visual aid and are available in many forms. Two spectacle lenses are customarily connected by a frame and form eyeglasses in common with it that can be positioned in front of two eyes in such a manner that one spectacle lens is arranged in front of each eye.
The function of a spectacle lens is based on the physical properties of an optical lens. A prerequisite is an optically transparent material. A front side and back side of the lens are manufactured in such a manner, in particular ground, that they form at least one optical focal point by convex and concave shaping. The determination of the focal point of the lens takes place as a function of a visual error of a person that is ideally compensated with the aid of the lens, during which in particular the erroneous natural focal point of the eye is corrected by the lens.
It can be necessary, especially for correcting presbyopia, that the lens must have several focal points. Depending on the distance of an object to the eye, the eye lens curves with differing strengths. In presbyopia this elasticity is limited - the eye lens hardens and thickens. This has the result that the visual error of the eye is different in the case of objects at a short distance to the eye (close vision) and the visual error in the case of remote objects (far vision). In order to be able to see objects sharply at all distances from the eye, the focal point of the eye must be corrected in accordance with the distance of the object from the eye by a certain focal point of the lens. The closest and classic solution is to change to a lens or to eyeglasses with a different focal point according to the distance of the object from the eye. However, it is also possible to shape the lens in such a manner that it has several focal points in different zones of the lens. It is possible by means of such a design that no changing of the lens is necessary and instead, by changing the direction of the view the zone of the lens is changed.
If a lens has two different focal points, as a rule one for the far vision range and one for the close vision range in the lower area of the spectacle lens, the lens is designated as a bifocal lens. Such glasses can usually be recognized by a visible transition between the two areas. To the extent that the focal point in a change of the angle of view, in particular from above downward, is frequently a different one, one speaks of a multifocal lens, where the focal point can also continuously be a different one.
Different methods are used to manufacture spectacle lenses. In the simplest case spectacle lenses are manufactured from flat blanks in several work steps. These blanks are plane disks of silicate glass or plastic. At first, in this method the desired geometries of the optical surfaces are produced on both sides of the lens by mechanical working (turning, milling, grinding), whereby the produced surfaces have a great surface roughness and are therefore optically non-transparent. Therefore, the machining is followed by a polishing process in which the surface roughness of the surfaces is decreased to such an extent that light is no longer inadmissibly scattered and the lens is therefore transparent.
Economical standard eyeglasses generally have standardized lenses today. At first, circular lenses are produced by casting or injection molding. The quality of such lens surfaces is so great that no further reworking of the surface is necessary. Only the contour must be adapted to a provided eyeglasses frame. Such changes have as a rule a spherical front side as well as a spherical back side. The term “spherical” denotes that the surface is a section of a sphere and that all points on the surface are at a distance radius R from a sphere middle point. Lenses with this type of construction have a few disadvantages. These standardized lenses are extremely economical to acquire but usually do not satisfy the desires for sharpness of vision in the near- and the far areas both over the entire angle of view from top to bottom and from left to right.
In order to reduce this deficiency of quality, more expensive spectacle lenses are usually individualized in that the eyes are measured and the calculation is made how a lens must be constructed in order to optimally eliminate an existing vision error. The result of the calculation is usually documented by an optician or eye doctor and transmitted in the form of a prescription to the person who manufactures the lens. Therefore, in the following a lens individualized according to prescription is designated as a prescription lens.
In order to produce prescription lenses different methods can be considered. For economic reasons in particular, methods have become popular that individually rework a raw lens manufactured on an industrial scale by non-cutting methods and polishing. Raw lenses of plastic are produced as a rule by casting or injection molding and raw lenses of silica glass are produced by lowering heated glass disks in forms. The quality of such lens surfaces, in particular the front side and the back side, is so high that basically no further reworking of the surface would be necessary. However, such a lens has one or more standardized focal points that frequently do not optimally correct the visual error of a certain eye. Therefore, the raw lens or partial areas of it, for example, only one side are individually adapted by reworking to the visual error of the certain eye. For this, a removal of material for shaping the prescription lens is calculated based on the front-side surface and back-side surface of the raw lens and of a prescription. The parameters of the surface geometry of the raw lens are obligatorily required for this calculation. A reworking on the front side and the back or only on one side can subsequently take place. A one-sided reworking generally results in lesser manufacturing costs than a two-sided one since the reworking expense is less. If the site is individualized by reworking, it is a so-called prescription surface.
The reworking takes place today with modern CNC milling machines, so that even complicated surface shapings can be realized. The reworking of a raw lens is, however, limited in that the stability of the lens must remain preserved and also the radii of curvature and the material strength of the raw lenses must impose limits to the changes. Therefore, at least a certain number of different types of raw lenses is required in order to be able to manufacture lenses for the largest possible visual error span. Lenses for rare and extremely strong visual errors that do not fall within this span must be manufactured by other production methods, for example, by production from a material block.
A production of individualized spectacle lenses from raw lenses is described, among others, in EP 0 744 646 A1. Different types of raw lenses are used in which one side surface has already been completed. They are produced by a casting- or injection molding process in which the front side of the raw lens is formed convexly and spherically. The prescription surface is calculated on the basis of a prescription issued by an eye doctor and subsequently manufactured by machining and polishing. It can be spherical, toric or multifocal. The desired optical properties of the spectacle lenses finally result in combinations of the standardized spherical front side and of the prescription side and correspond to the given prescription.
However, significant disadvantages result from a prescription manufacture according to EP 0 744 646 A1. As a result of the spherical front side the back-side prescription surface is very flat in the lower spectacle lens area that should correct the visual error in the close vision range. Even convex curvatures can be required on the back side of the spectacle lens in order to eliminate the visual error. Wearers of the eyeglasses perceive this as being extremely unpleasant and disturbing. In particular, this leads to another optical error: The more obliquely an eyeglasses wearer looks on the back-side surface of the lens, the greater the prismatic effect of the lens. This causes distortions since light of different wavelengths is refracted with different strengths. The back-side surface of the lens should accordingly be shaped in such a manner that the lens surface stands vertically to it in every direction of viewing. That is, the back side of the lens must be concave. However, this central requirement is not met by EP 0 744 646 A1.
A production of individualized spectacle lenses is also described in DE 197 01 312 A1. It describes a raw lens that is reworked exclusively on the back side. That means that the prescription surface is on the back side of the lens. The front side, on the other hand, remains unchanged and has a spherical or an aspherical front side. An aspherical surface is characterized in that it can be described by a mathematical function according to DIN ISO 10110-12, whereby the radius continuously changes from the middle axis outward. The back side of the lens is calculated according to prescription as a multifocal freeform surface and is produced by reworking.
Even DE 101 03 113 A1 describes a rotationally symmetrical and in particular spherical front side of the lens without, however, explaining it further.
Furthermore, a raw lens is known from EP 1 990 676 A2 that is bisymmetrical and for which only the back side should be reworked. The front side of the raw lens is formed by two partial surfaces. Whereas the main surface is a sphere, a toric surface is arranged decentrally in the main surface. This toric surface has a lesser curvature than the main surface and should later be arranged in the lower area of the spectacle lens, in particular in the area of the spectacle lens that should correct the visual error in the close vision range. Toric surfaces and toric glasses serve not only to correct a shortsightedness or farsightedness but also to compensate a curvature of the cornea (astigmatism). This construction is more complicated. Toric glasses can be recognized in the prescription in that the strength is noted in three parts. The individual parts are designated sphere (sph), cylinder (cyl) and axis.
It is disadvantageous in EP 1 990 676 A2 that the prismatic effect is not taken into account in the horizontal direction. The blurs resulting from this in the viewing directions left and right and temporal and nasal, are disturbing not only for the wearer of the eyeglasses but rather even dangerous, e.g., in the case of a vehicle driver who looks back over his shoulder with such eyeglasses, for him and third parties. Wearers of eyeglasses frequently also suffer from nausea, headaches and feelings of dizziness on account of this blurriness.
Furthermore, the esthetics of the spectacle lenses is adversely affected since the transition from the spherical to the toric surface on the front side of the lens can be perceived in particular by reflections of light. Moreover, the bisymmetry requires a very precise rotational alignment of the raw lens in the tool before the working of the prescription surface can take place. If this does not happen, the worked surface is not at the location that results according to calculation in an optimal correction of the visual error. The lens would then be unusable.
DE 1 422 125 A teaches another shaping of the front side. This side has an aspherical and constantly changing surface. In particular, the asphere is described by an involute that is constructed with an evolute. Such an aspherical front side for spectacle lenses described by an involute is also described by DE 1 115 609 B. This involute is generated by an evolute composed of several pieces of circular arcs. Even in such a construction the surface of the front side of the raw lens constantly and continuously changes.
A further forming of the surface of a spectacle lens raw lens is described by US 4 484 804 A. Here, the surface is formed by two spheres with different radii. In particular, an area of the surface that should subsequently be in the close vision area of the spectacle lens is formed by a sphere with deviating radius. In order to prevent a visible edge in the transition between the first and the second asphere the transition is smoothed.